Impact of a coupled ocean wave-tide-circulation system on coastal modeling Il-Ju Moon Graduate School of Oceanography, University of Rhode Island, South Ferry Road, Narragansett, RI 02882, USA * Corresponding author. Tel.: +1-401-874-6508;fax: +1-401-874-6728. E-mail address: [email protected] (I.-J. Moon) Submitted to Ocean Modeling (Status: New) Abstract The impact of a coupled ocean wave-tide-circulation system on coastal modeling for wind-waves, oceanic circulation, and water-mass simulation is investigated by coupling of two well-tested models: the third-generation wave model (WAVEWATCH-II) and the Princeton ocean model (POM). In this study, several numerical experiments in the Yellow and East China Sea (YECS) are performed for the ideal winter case and the typhoon Winnie case. In the coupled system, wind-waves are influenced by both currents and sea level elevation induced by tides, storm surges, and oceanic circulation. Tides were the most influential factor in modulating mean wave characteristics in the YECS. The magnitude of the modulation of mean wave parameters at neap tides is found to be half that at spring tides. In the YECS the tides affect not only wind-waves, but also seasonal circulation and water-mass distributions. Tides increase the bottom friction of the YECS significantly and this contributes to a change of winter current direction up to 60°C in the YECS and a decrease of surface temperatures along the trough of the Yellow Sea up to 4°C in winter. Tides in summer produce the strong vertical mixing in shallow regions. This leads to the formation of tidal fronts in a boundary between well-mixed and stratified regions and causes sea surface temperatures (SST) along the west coast of Korea decrease as much as 3°C. Effects of ocean waves on coastal circulation and SST simulations are investigated under typhoon Winnie conditions. The results show that young waves generated in the rear left quadrant of the storm track enhance sea surface stress and this contributes to the change of SST up to 0.5°C. 1 Keywords: circulation; coupling; interaction; oceanic model; storm surges; tides; waves; wave model 1. Introduction In coastal and shelf regions, tides, storm surges, wind-waves and oceanic circulation are the main processes that determine the physical environment of the region and are major components of marine forecasts. It has been known that these processes coexist in many cases and their interactions are very important for individual dynamic processes (Longuet-Higgins and Stewart, 1960; Prandle and Wolf, 1978; Janssen, 1992; Bao et al., 2000; Welsh et al., 2000). Although there have been a number of studies on the individual processes for the last decades, how theses processes influence one another is still an important issue in coastal modeling. It is believed that wind-waves have a significant influence on sea surface stress (or roughness) that generates currents in storm surge and coastal circulation modeling (Smith et al., 1992; Janssen, 1992; Mastenbroek et al., 1993). From measurements in the North Sea, Smith et al. (1992) found that the sea surface roughness decreases as wave age increases. Janssen (1989, 1991, 1992) pointed out that wave-induced stress in young wind seas might be a substantial amount of the total stress in the surface layer, resulting in a considerable enhancement of the drag of airflow. Based on the theory of Janssen (1991), a number of storm surge models have considered the effect of ocean waves on storm surge predictions (Mastenbroek et al., 1993; Zhang and Li, 1996, 1997). Storm surges, tides, and oceanic circulations, on the other hand, have a significant effect on a wave field if the effects of their currents and elevations are big enough. 2 Tolman (1991) described the effect of tides and storm surges on wind-waves for three North Sea storms cases and showed that tides and storm surges should be considered as unsteady mediums for wind-wave propagation if wave-current interactions are assessed. Holthuijsen and Tolman (1991) and Komen et al. (1994) indicated that oceanic circulations such as the Gulf Stream and the Agulhas current also influence wave fields of the regions. In coastal areas with strong tides, tides are an important factor affecting storm surges and ocean circulation. Tang et al. (1996) demonstrated that a nonlinear interaction between the storm surge and the tides affects strongly the prediction of coastal sea levels and is predominantly caused by the quadratic bottom friction. Lee (1999) emphasized the importance of M2 tide in the circulation of the East China Sea. Simmons et al. (2003) suggested that the incorporation of tidal mixing parameterization into the ocean general circulation leads to improving the representation of the ocean mixing processes. Schiller (2004) pointed out that tidal currents play an essential role in transport and mixing processes in the Indonesian Seas. Because of the importance of such environmental interactions, accurate coastal modeling strongly needs the dynamically coupled system of two or more processes. Recently, several coupling experiments in coastal areas have been studied (Mastenbroek et al., 1993; Zhang and Li, 1996; Tolman, 1991). Their main interests were interactions between the storm surges (or tides) and wind-waves using a coupled storm surge and wave model. Mastenbroek et al. (1993) and Zhang and Li (1996) investigated wave effect in aspects of storm surge modeling; Tolman (1991) described the effects of tides and storm surges on wave modeling. However, these were not fully coupled coastal modeling 3 studies because their storm surge models did not consider coastal circulation or variations of water temperature and salinity. In some cases, the coupling was only one-way. Water temperature, especially sea surface temperature (SST), is an important factor to be considered in coastal coupling studies because it is very sensitive to the dynamic response of air-sea interaction and consequently affects both the atmospheric and oceanic systems. Impact of SST on hurricane intensity forecasts is a good example that emphasizes its importance in the air-sea interaction. Bender and Ginnis (2000) show that a decrease of SST due to entrainment of the cooler waters from the thermocline in the mixed layer under hurricane wind forcing produces a significant change of storm intensity. SST changes are strongly dependent on the surface momentum flux and an upward enthalpy flux across the interface in the upper ocean, which are determined either by changes in the atmospheric forcing and changes in the ocean circulation. Therefore, SST can have large variability on a wide range of temporal spatial scales resulting from changes in tides, ocean waves, and storm surges. A fully coupled system of atmospheric model, wind-wave model, and ocean circulation model is the ultimate goal in environmental modeling research and operational forecasting. Bao et al. (2000) studied air-sea interactions with such a system for the first time. They successfully introduced the roles of ocean mixing, sea spray and sea-surface waves on air-sea interaction under high wind conditions with the coupled atmosphere-wave-ocean modeling system, although they left further studies on two-way coupling system between an ocean circulation model and a wind-wave model, which is not included yet in their model. Indeed, the coupling of ocean circulation and wind-wave model is involved with several important dynamics at the ocean surface and bottom 4 boundaries, such as, air-sea momentum flux modifications (induced by waves and sea spray), wave-current interactions, and upper ocean (or bottom) mixing processes (induced by tides, storm surges, Stoke drift, Langmuir circulations, and breaking waves). While these processes are well studied for the last decades, there are still many uncertainties in estimating quantitatively the effects, which have contributed to making a full coupling of ocean circulation and wind-wave model difficult. In the present study, we developed a coupled ocean wave-tide-circulation system, which takes into account major physical features of coastal seas (such as realistic tides, wind-waves, storm surges, oceanic circulation, surface heat flux and fresh water input) and major coupling processes (such as wave-induced surface momentum exchanges, wave-current interactions, and tide-induced mixing processes). This study does not cover all mechanisms involving the coupling of ocean circulation and wind-wave model, but focuses on investigating interactions among well-established major processes in coastal regions and how the interactions affect the realistic coastal simulation. The first part of this paper examines wind-waves interacting with tides, storm surges, and oceanic circulations. The impact of these interactions on the coupled system is investigated through a series of numerical experiments in the Yellow and East China Sea (YECS). In the second part, the effect of tides and wind-waves on the circulation and water-mass distributions of the YECS is evaluated within the coupled system. The models used in the coupled system and their coupling processes are described in section 2. A detailed description of model areas, experimental designs, and data sets for numerical experiments are given in section 3. Effects of the coupled system on wind- wave and coastal circulation simulations are investigated in sections 4 and 5. A summary 5 and conclusion are given in the last section. 2. Coupled ocean wave-tide-circulation system 2.1. Model descriptions The coupled ocean wave-tide-circulation system used in this study is based on the synchronous coupling of two well-tested models: a third-generation wave model (WAVEWATCH-II) and a Princeton ocean model (POM). WAVEWATCH-II (WW) was developed at the NASA Goddard Space Flight Center in the spirit of the WAM (WAMDI Group, 1988), but is designed with more general governing transport equations allowing incorporation of unsteady and inhomogeneous currents on ocean waves, which permit full coupling with ocean models.